Solar System/Terrestrial Bodies

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This page is outdated. The information it provides needs to be updated to reflect changes in the event rules or recent scientific developments. Please keep this in mind when reading the page and add updated information if possible.
This page needs to be updated to reflect the 2019 rules.
For general information about this Division B event, see Solar System.

Terrestrial Bodies is the topic of the Division B event Solar System for the 2018 season. The first part, a written test, covers the terrestrial planets (excluding Earth), the Earth's moon, Phobos, Deimos, and Io, the asteroid belt, and near-Earth asteroids. The second part, a hands-on/interpretive task, covers the geologic characteristics and evolution of the rocky bodies. The specific focus of the event is the history, formation, and geology of these rocky bodies.



Mercury is the smallest planet in the solar system with a radius of about 2439 km. It is also the planet closest to the sun at an average distance of about 57.9 million kilometers, or about 0.39 AU. (Mercury has a highly elliptical orbit.) The length of a year on Mercury is 88 Earth days long, and a day on Mercury is 59 Earth days long. The surface gravity of Mercury is 1/3 of Earth's, so it can only hold an extremely thin atmosphere consisting of small amounts of hydrogen, helium, and oxygen. Mercury’s surface is scarred with the craters of impacted meteors. The surface temperatures at day and at night are very different, because of the thin atmosphere. The temperature during the day is 227°C and the temperature at night is -173°C. The most noticeable feature is the largest impact crater on its surface, the Caloris basin.


Venus is the third smallest planet in the solar system with a radius of 6051 km. It is also the second closest to the sun at a distance of 108.2 million kilometers. It is the only planet whose day is longer than its year. Its day is 243 Earth days, and its year is 225 Earth days. The extremely long rotation periods of Venus and Mercury is thought to be caused by mutual gravitation pull; Mercury is sometimes thought to be an escaped moon of Venus. Venus is often called Earth's sister planet because of its close proximity to Earth, and because of its similar diameter and mass. People even thought it could hold life, but sadly people discovered that the greenhouse effect on Venus raised the surface temperature to the highest in the Solar System.


NOTE: Earth is not included as a major part of the event for 2018; however, a basic knowledge of how Earth fits in the context of the Solar System will likely be of use.

Earth is the third planet from the Sun. It is also the fifth largest planet at a radius of 6378 km, ranking below the gas giant planets. It is the only planet in the universe currently known to support intelligent life. It is used as a basis for many measurements of planets and other things in the solar system, for example, the AU (Astronomical Unit), the average distance between the Earth and the Sun, 93,000,000 miles or 149,600,000 km. The year on Earth is equal to about 365.256 Earth days, and a corresponding day is equal to one 1 earth day, hence the definition.


Mars is the fourth planet from the Sun at a distance of about 227,392,000 km, with a highly elliptical orbit. It is often called the Red Planet due to the large quantities of iron oxide present in its soil. The Romans saw it as blood, so they named it for their god of war, Mars. It is the second smallest planet, with a radius of 3397 km. It has a day length of about 25 hours, and a year equal to 687 Earth days. It has been suggested that Mars may hold life, but it has not been proven. Mars contains two polar ice caps consisting of frozen ice and carbon dioxide located on the southern and northern poles.

30% of the polar ice caps are made up of dry ice or CO2. The dry ice then sublimates creating large gusts sweeping across the polar ice caps creating cirrus clouds in the atmosphere. Mars has two tiny moons, Phobos and Deimos, believe to be captured asteroids.


Earth's Moon

Earth's Moon is known as the Moon. It is Earth’s only known permanent natural satellite. It takes 27.5 Earth days for the Moon to fully orbit the Earth. The moon is widely thought to have formed 4.51 million years ago, not too long after Earth, supposedly after a Mars-sized body collided with Earth. The moon is in synchronous motion with Earth, and always shows the same face, although about 59% of it can be seen over time due to libration.

The moon appears the same size in the sky as the sun, resulting in total solar eclipses, but the moon’s distance from Earth is slowly increasing.


Io is the third largest of Jupiter's four Galilean moons, and is the most volcanically active body in our solar system, with over 400 volcanoes, the result of tidal heating from the pull of Jupiter and its other moons. It is the innermost of the four Galilean moons, and has the highest density of all the moons.Io was discovered in 1610 by the Italian scientist Galileo. Its largest volcano, Loki Patera, has a diameter of 202 kilometers (126 mi), and is one of the few moons to contain an active lava lake. During an eruption, a wave of foundering crust spreads out across the patera at the rate of about 1 kilometer (0.6 mi) per day, until the crust of the lake has been resurfaced.

Phobos and Deimos

Phobos and Deimos are the only two known moons of Mars, discovered in August 1877 by astronomer Asaph Hall and named after Phobos and Deimos from Greek mythology. Phobos is the inner moon and seven times as massive as Deimos, the outer moon. Phobos has an average diameter of 22.2 km. It takes approximately 8 hours for Phobos to complete an orbit around Mars, and its surface gravity is only about 0.0057 m/s squared. Phobos orbits only about 9377 kilometers away from Mars, compared to Earth's moon, which orbits 384,400 kilometers away from Earth. Deimos is much smaller, with an average diameter of only 12.6 km. Both moons are usually believed to be captured asteroids, but their origin is still controversial. Phobos and Deimos resemble C-type asteroids, with albedos, densities, and spectra similar to those of C-type asteroids. Another theory is that Mars was surrounded by many such objects, but many were ejected, or that the moons may have been a result of a large collision between Mars and a smaller protoplanet.

Phobos, however, won't zip around Mars forever. The doomed moon is spiraling inward at a rate of 1.8 centimeters (seven-tenths of an inch) per year, or 1.8 meters (about 6 feet) each century. Within 50 million years, the moon will either collide with its parent planet or be torn into rubble and scattered as a ring around Mars. The most dominant feature on Phobos is large impact crater dominates the moon. Stretching nearly 9.5 km, Stickney Crater covers most of the surface.

It is possible for Mars to have moons smaller than 100 meters in diameter and a dust ring between Phobos and Deimos, but neither have been discovered so far.

Small Bodies


Asteroids, sometimes called minor planets, are leftover remnants from the formation of the solar system. There are three major types of asteroids, C-, S-, and M-types. The C-type is the most common, consisting of clay and silicate rocks, and are usually dark in appearance. S-type asteroids are made of silicates and nickel-iron. Finally, the M-type asteroids are metallic, and contain the largest amount of nickel-iron.

Most asteroids are found in the asteroid belt, a donut-shaped region between Mars and Jupiter. Asteroids have a wide variety of sizes, from the largest, Ceres, at about 946 kilometers in diameters to tiny bodies smaller than a house. Only the largest asteroids are not irregularly shaped. Also, asteroids can have highly elliptical orbits, and some even have companion moons. Jupiter's high gravity can often alter asteroids' orbits and knock them out of the main asteroid belt.

A particularly important category of asteroids are Near-Earth asteroids, or asteroids that approach Earth's orbital distance, and pose an impact threat.


A "sand- to boulder-size" piece of space debris. The official definition from the IAU is "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom". Traditionally, anything smaller than 10 meters across is considered a meteoroid, while anything larger than 10 meters is an asteroid. Once a meteoroid enters the atmosphere of Earth or another planet, it is considered a meteor. If it reaches the ground and stays (more or less) intact, it's called a meteorite. A method to remember this is meteoroid is in the void of space and a meteorite is right here.


A small solar system body that has a coma (the dust particles gathered around the comet's nucleus that give it an "atmosphere") and/or a tail. The nucleus itself is made up of water ice, dust, frozen gases and small rocky particles. The nuclei range from 100 meters across to more than 40 kilometers. As the comet approaches the sun, solar radiation cause the gases inside to vaporize and carry the dust with them. The gases also become excited by sunlight and emit electromagnetic radiation. Comets leave a trail of solid particles behind them, and if a comet crosses earth's path, there will most likely be meteor showers when earth passes through the debris field. For example, Halley's Comet causes the Orionid Showers and the Swift-Tuttle Comet causes the Perseid showers.

Short-Period Comets- Comets with an orbital period of less than 200 years. Their orbits are in the same direction as the planets, close to the ecliptic, and their aphelion is generally in the area of the outer planets. They are divided into the Jupiter family (orbital period less than 20 years) and the Halley family (orbital periods between 20 and 200 years).

Long-Period Comets- Comets with orbital periods of more than 200 years, sometimes even thousands or millions of years. Their orbits are very eccentric, often don't lie near the ecliptic, and their aphelion is far beyond the outer planets. However, all long-period comets are still gravitationally bound to the sun; comets that have been ejected from the solar system by the gravity of the outer planets are no longer considered to have an orbital period.

Sungrazing Comets- Comets that have a parabolic or hyperbolic trajectory, i.e. their trajectories only let them enter the solar system once (hence the name). Other than that, they are very similar to long-period comets. The large sungrazers often break up into chunks while smaller ones can disintegrate (e.g. comet ISON)

Geologic Characteristics

This section is incomplete. It does not cover all important aspects of this subject. Please keep this in mind when reading the page and add relevant information if possible.

The second part of the competition will ask competitors to complete one or more hands-on or interpretive tasks from these topics:

History and Formation of Rocky Bodies

The leading theory for the formation of the moon is from a collision of a Mars-sized body with the Earth.

Surface Dating- In order to approximate the age of a surface, many would use the amount of creating or weathering on the planet. The more craters a planet has, the older it probably is. Another way to look at this is based on whether or not it is still geologically active. However, there are some surfaces proven to be geologically young by radioactive dating, that look very old based on this method. An example of this is Venus. It is puzzling that it is geologically young, because it has no evidence of plate tectonics or other forms of surface recycling. Radioactive dating is often used on earth to identify fossils or prehistoric remains, but if samples of dirt come back, like from the moon, radioactive dating can be used to get an exact date on the surface age.

Compositions of Rocky Bodies

Geographic Features

Regolith- Regolith is a layer of unconsolidated material on top of solid rock, like moon dust. On Earth, this is soil.

Volcanism, Weathering, and Cratering

Extraterrestrial volcanos form differently on other planets than they would on earth, as the other terrestrial planets are not large enough to sustain plate tectonics, Earth just barely making the cut. Planetary volcanism is not always hot. Although most volcanos on earth are 800-1200°C Silicate Magma, extraterrestrial volcanism can be of mud, ice, carbonate, and more. Whether or not there is volcanism on a planet can depend on planet size, core size, and internal temperature. Really Great Resource for Extraterrestrial Volcanism

Planetary weathering by meteorites, micrometeorites, cosmic winds, and solar rays is directly responsible for the formation of regolith. Galactic cosmic rays and solar cosmic rays may speed up the weathering of rocks on airless bodies. Irradiation is basically indirect radiation or the process of being exposed to radiation. Sputtering is when particles excreted from the sun hit celestial bodies. The other thing that it may come from is the bombardment of meteorites and micrometeorites.In-Depth Article On It

Remote Sensing and Satellite Imagery

Motions of Terrestrial Bodies

This section is incomplete. It does not cover all important aspects of this subject. Please keep this in mind when reading the page and add relevant information if possible.

Rotation and Revolution

Libration, Phases, and Eclipses

See also: Solar System#Eclipses


Libration is the apparent occilation of an orbiting body, most notably the perceived movement of the Moon as seen from Earth. Lunar libration causes slight changes in the Moon's apparent size, and as a result, observers on Earth can see about 59% of the Moon over time, despite tidal locking.

Kepler’s Laws

See Solar System#Kepler's Laws of Planetary Motion for more detail.

Gravitational Effects of the Moon

Missions to Terrestrial Bodies

See also: Solar System/Missions